One of the benefits of a using a MOSFET device is the high rate at which the device can be switched from an “on” state to an “off” state. Faster switching allows for a more efficient device, but it also creates undesirable waveforms that may negatively affect the device. Specifically, a MOSFET device may be under high stress because the maximum voltage and maximum current must be supported simultaneously. This may cause the device to exceed the safe operating area (SOA) and result in device failure. Therefore snubbers, such as a resistor-capacitor (RC) snubber, or a resistor-capacitor-diode (RCD) snubber, are often used to improve the switching waveform in order to reduce the peak voltage and current. A typical RC snubber circuit 100 integrated with a MOSFET device 110 is shown in FIG. 1A. In the snubber circuit 100 a snubber capacitor 118 and snubber resistor 119 are connected in series with each other and in parallel with the MOSFET device 110. In the MOSFET device 110 the capacitor 117 is the source to drain capacitance CDS inherent in the device. Additionally, the node 114 is maintained at gate potential, node 115 is maintained at drain potential, and node 116 is maintained at source potential.
A cross sectional view of a MOSFET device 110 that incorporates a snubber circuit is shown in FIG. 1B. In the figure the MOSFET device is a shielded gate trench (SGT) MOSFET device. It should be noted that the electrical connection between the source 112 and the shield electrode 113 (in addition to the snubber resistor 119) are not visible in this cross sectional view, and therefore are represented with schematic connections. A typical SGT MOSFET device has a trench lined with an insulative material 126. The insulative material 126 also electrically isolates the shield electrode 113 from the gate electrode 124. A source contact 146 may connect the source material 112 to the source region 128 that is formed in the body layer 127. A drain contact 129 may also be formed on a bottom surface of the device 110.
However, despite the benefits of having a snubber circuit integrated with a MOSFET device, the presence of the snubber resistor 119 may actually decrease the breakdown voltage VBD of the device. For example, during the turn-off of an SGT MOSFET device 110, the presence of a snubber resistor 119 causes the shield voltage to increase above the source voltage because of displacement drain current going through shield resistor. The difference in the potentials between the shield electrode 113 and the source 112 may cause a reduction in VBD. The device may breakdown during switching and hence reduces the reliability of the device.
Therefore, there is a need in the art to provide a snubber circuit that has a higher resistance during the beginning of switching to reduce displacement current and thereafter has a low resistance near the middle of switching in order to get have an optimized waveform without sacrificing VBD.
It is within this context that aspects of the present disclosure arise.